Atlas of Siliceous Hot Spring Deposits (Sinter) and Other Silicified Surface Manifestations in Epithermal Environments

Abstract

Sinters, or siliceous hot spring deposits, are the terrestrial surface expressions of predominately low sulphidation epithermal systems. Sinter deposits form at the intersection of the water table and the Earth’s surface, where near-neutral pH alkali-chloride thermal fluids, oversaturated in silica, are expelled and chemically precipitate hydrothermal minerals and amorphous silica on biotic and abiotic surfaces. The silica precipitate has been observed in Archean to present day settings as bedded, erosion-resistant sinter mounds and sheet deposits, metres to tens of metres thick and exhibiting a variety of sedimentary lithofacies. Lithofacies of siliceous sinter are produced by thermophilic microbes, and other taxa, that flourish along gradients of pH geothermal fluid flow rate, fluid composition and temperature (from ~100 °C to ambient). Physicochemical processes relevant to the depositional environment also may be indicated, such as wetting-drying cycles, interaction with local water bodies and seasonality. In particular, the decrease in temperature by cooling and evaporation with distance from the spring vent source area produces a predictable bio- and lithofacies gradient that is recorded in the sinter chemical precipitate. Sinter lithofacies also may archive indications of the relative volume of spring discharge as well as its flow direction. Identification of sinter lithofacies in the geologic record allows their differentiation from other silicified features often misconstrued as sinter. This report, in the style of a comparative, visual and descriptive atlas, describes siliceous sinter lithofacies from vent and proximal slope (~100–65 °C) through middle and distal apron areas (~65–30 °C), to the commonly widespread, geothermally influenced marshes (tepid to ambient). Geometry, thickness and textures of sinters also are utilised to help infer the formational environment, temperature, relative fluid volume, flow direction and lateral extent of each of the lithofacies. These characteristics together can be used to pinpoint geothermal up-flow areas, identify more distal outflow areas and assist in characterising a geothermal system, thus potentially aiding delineation of zones of epithermal mineralisation. Furthermore, other silicified volcanic and sedimentary deposits may exhibit textures that are similar to sinter and which have been misidentified as such. These are known as pseudosinters. A variety of pseudosinters common in epithermal surface environments also are described here to aid recognition and distinction of pseudosinter and sinter. Such differentiation may enhance the potential for identifying vectors to epithermal up-flow, and for characterising extremophile (paleo)communities preserved within these extreme (paleo)environments throughout the geologic record.